Many prior art electronic thermometers employ thermistors as transducers for determining the temperature of the environment whose temperature is being measured. In clinical thermometers, a thermistor is attached to a probe member of the thermometer, and the probe is inserted into a body cavity such as the mouth of a human being whose temperature is being measured. Once inserted the probe must remain in place for a sufficient time to allow the body tissue to heat the thermistor and probe to the temperature of the body tissue. Until the probe reaches a temperature which is equivalent to the body temperature, the probe remains at a lower temperature than that of the individual. Measuring the temperature before the probe has reached a temperature equalized with that of the environment will result in an inaccurate temperature reading, typically a low temperature reading.
The amount of time that the probe must remain in the body cavity or in any other environment depends on the thermal time constant of the probe. The thermal time constant is a measure of the time required for a particular object to absorb or expel a quantity of heat, and therefore represents a measure of the speed with which heat energy can be transferred by the object. Typically, the time constant of the thermistor is much less than the time constant of the probe, on the order of ten times less, but because the thermistor is thermally attached to the probe the time required for the thermistor temperature to equalize to that of the environment is essentially dependent upon the thermal time constant of the probe.
Different prior art techniques have been utilized to predict the final temperature before the thermistor has actually reached a temperature equalized with that of the environment. Electronic thermometers of this type are known as predicting thermometers and have been typically used to attempt to reduce the time necessary for accurately taking temperature of human beings. The prior art predicting thermometer may employ an electronic race circuit or some other type of compensation circuit which attempts to offset the actual measured temperature by a factor intended to compensate for the thermal time constant of the probe. The problems with predicting thermometers are that these compensation arrangements do not accurately simulate the individually different physiological characteristics and responses of various different people. Predicting thermometers also present problems in their calibration. Typically, the thermometer is calibrated in an oil or water bath which has a much better thermal conductivity than the mouth or other body cavity. The compensation circuits react differently in oil or water baths than in the body cavity of the human being to make accurate calibration for human use difficult or impossible.
A direct reading thermometer is one which directly measures the temperature without use of compensation or predicting arrangements. The well known mercury thermometer is one example of a direct reading thermometer. Direct reading thermometers can be accurately calibrated since the temperature they measure is the same temperature as the environment. Of course, the disadvantage of the prior art direct reading thermometer is that it requires a predetermined time of contact with the environment during which the temperature of the thermometer equalizes with that of the environment.
Other limitations and disadvantages of prior art electronic thermometers are known and appreciated, and limited solutions to some of these problems may have been achieved. In general, however, the various factors, problems, limitations and disadvantages present in the prior art should become more fully appreciated and recognized in light of the improvements and teachings of the present invention.